The present invention, in some embodiments thereof, relates to the field of wireless communication and, more specifically but not exclusively, to point-to-point communication systems and, yet more specifically but not exclusively, to single carrier continuous transmission point-to-point communication systems.
A non-limiting example of a point-to-point communication system can be a communication system such as employed by cellular backhaul networks or other wireless networks that use microwave and millimeter waves. These communication systems transmit continuously and are often required to provide a guaranteed quality of service. The quality of service may for example be measured in terms of delay, bit error-rate or frame error-rate. In order to guarantee a required quality, care must be taken in designing the network with respect to channel conditions and interference.
In order to minimize interference, high-gain antennas are used and the communication frequencies are regulated. Each point-to-point link in a network is assigned a pair of frequencies. The frequencies are known as an uplink (UL) frequency and a downlink (DL) frequency. One of the two nodes in the point-to-point link transmits using the UL frequency and the other transmits using the DL frequency. The bandwidth allocated to both nodes is identical.
Channel conditions may change over time, due to multipath fading and/or rain attenuation. However, the channel conditions are typically the same for the uplink and the downlink, since the UL and the DL frequencies are relatively close. For example, the UL frequency may be 18 GHz and the DL frequency may be 18.5 GHz.
In case channel conditions deteriorate, and a receiver is unable to provide the required quality, a corresponding transmitter increases transmit power until the link reaches some predetermined quality level. If performance is still not good enough, it is common to switch the communication system to a mode, a different error code and/or a different modulation, which is more resistant to noise. In such a case the capacity provided by the link is typically reduced.
In QAM modulations, switching to a more noise resistant mode may mean using a smaller constellation, for example, switching from 64 QAM to 16 QAM or even to QPSK (4-QAM). Alternatively, switching an error-coding scheme may reduce sensitivity to noise and/or to channel deterioration, at the expense of bit rate.
To summarize, current systems are allocated equal-bandwidth channels for the uplink and downlink, and they may change the transmit power or the constellation in both uplink and downlink in the same fashion when dealing with fading in the wireless channel.